The present invention relates to a container tightness tester with a conveyor and with a plurality of test stations, each for at least one container, with at least one pressure sensor at each test station, which is operationally connected on the output side with the input of an evaluation unit.
Testers of the above type are known in which, as viewed on the carousel, the respective test stations are pressurized in a first rotation angle position of the carousel, in the case of open containers, their interiors, or in the case of closed containers, corresponding test chambers at the test station, and in which the pressure that depends on the tightness of the container under test is detected in at least one additional predetermined rotation angle position of the carousel and then evaluated. This known method and/or this type of tester are disadvantageous for a number of reasons. Because pressure detection is performed by the individual test chamber while passing through a certain rotation angle, generally for example a certain position for linear conveyors, only a limited time is available for pressure measurement, as a function of the rotational speed or of the speed in general, and hence of the throughput rate. This limits the rate at which containers can be tested per unit time and is especially problematic in continuous carousel operation. In addition, it is not possible to follow over time the pressure that depends on leakage because the pressure sensors provided perform their measurements only at certain times.
Systems for extremely reliable tightness testing therefore have at least one pressure measurement sensor permanently associated with each of the test stations that go around with the carousel, as well as likewise permanently assigned evaluation electronics that allow testing to be performed at the test stations freely throughout the entire time interval, during which a container loaded into a test station on the carousel goes around with the carousel.
This latter procedure and/or the corresponding systems admittedly have an extremely high detection accuracy but they are also correspondingly expensive in that they are autonomous as mentioned above, with each test station being equipped with the necessary evaluation electronics.
The goal of the present invention is to provide a container tightness tester of the species recited at the outset in which, on the one hand, the detection accuracy with respect to the latter systems is reduced only insignificantly if at all, but this can be accomplished at a much lower expense. For this purpose, the system of the species recited at the outset is characterized by a common evaluation unit provided for several of the pressure sensors and, in addition, a multiplexer unit clocked by a timer is connected between one input of the common evaluation unit and the outputs of pressure sensors.
In testing the tightness of filled containers, especially those filled with a liquid filing, as described in detail in the additional U.S. application Ser. No. 08/944,183, now U.S. Pat. No. 5,962,776, issued Oct. 5, 1999, filed at the same time as the present application (a copy of which application was filed herewith as Attachment A), there is the problem that when there is a vacuum in the vicinity of the container, it is difficult to detect a leak in the parts of the wall that are contacted by the liquid filling. The liquid that escapes to the outside in this case has a practically self-sealing effect. A reliable tightness test for such containers is only guaranteed if a leak occurs in a wall area opposite an air inclusion inside the container. For this reason, in the abovementioned application (Attachment A) filed at the same time it is proposed simultaneously with testing of tightness by observing the pressure in the environment of the container, to perform an impedance measurement directly at the wall of the container, in view of the fact that escaping liquid immediately causes a change in impedance at a measurement point located between at least one pair of measuring electrodes.
With this in mind, it is now proposed to provide at least one pair of electrodes in a receiving chamber for at least one container when testing containers filled with liquid filling, said electrodes being spaced and exposed and provided centrally for all test stations, and connected operationally in turn by a multiplexer unit with the respective electrode pairs. This makes it possible to provide both an evaluation unit for pressuredetecting testing as well as an evaluation unit for central impedance detection testing for all the test stations provided on the carousel and to multiplex the respective pressure sensor and impedance measurement section outputs on a time-staggered basis to the corresponding evaluation units over time.
In another preferred embodiment of the evaluation unit that is connected to the electrodes and the evaluation unit that is operationally connected with the pressure sensors, one and the same central evaluation unit is used. This is readily possible in that pressure sensors usually deliver a voltage signal, especially during DC resistance measurement as impedance measurement. A measurement circuit equipped with the variable resistance to be measured, such as a voltage divider, can likewise be readily designed so that the resistance-dependent output signal is a voltage signal. By virtue of this procedure, the cost of simultaneous pressure and impedance testing is especially low.
The possibility is also provided for implementing the multiplexer units connected between the impedance measurement sections and the evaluation unit on the one hand and the multiplexer units provided between the pressure sensors and the evaluation unit on the other hand by a single common correspondingly time-controlled multiplexer unit, which switches the number of inputs that corresponds to the pressure sensor and impedance measurement sections to a single output for the central evaluation unit provided.
In WO94/05991 of the same applicant, as the present application, a pressure tightness measurement method is explained in detail in which the output from a pressure sensor is connected at a first point in time to both outputs of a differential unit. The possibly amplified output signal from the differential unit is interpreted as a zero-offset signal and stored. During pressure detection at a second point in time, the previously stored zero-offset signal is connected as a zero-compensation signal making it possible with high amplification to evaluate the corresponding pressure differential-signal electrically.
This procedure can also be used in the system according to the present invention, with the evaluation unit being so designed that an input signal-dependent signal that appears at the first point in time is stored as the zero-reference signal value and later supplied as the zero compensation signal. At a second, subsequent point in time, an additional input-dependent signal, possibly amplified, is evaluated as the evaluation signal relative to the compensated zero value as a differential. This procedure can be used for both evaluations when pressure sensor evaluation and impedance evaluation are present at the same time, since, as far as impedance measurement is concerned as well, which after all depends on an impedance difference measurement, the impedance difference that arises can occur with reference to the exact compensated zero reference.
The invention will now be described using the figures as examples.